The present invention relates to a communications network of a packet switching system, and more particularly to technology of a UPC unit which is subject to a feedback limitation such as an ABR traffic.
As regulated in Shirish S. Sathaye, "The ATM Forum Traffic Management Specification Version 4.0", ATM Forum/95-0013R13, April, 1996 (TM4.0), the ABR service used in the ATM network is executed to report an allowable cell rate (ACR) from the network to a terminal so that the terminal can send out data at the ACR or lower rate. In general, the network for supporting the ABR service is operated to report a smaller ACR to the terminal if a traffic congestion takes place in the network or a larger ACR to the terminal if the traffic congestion does not take place. This is because the reduction of the traffic flown to the network leads to overcoming the congestion if the network is in congestion or the network may accept more traffics if the network is not in congestion.
The traffic direction from the network to the terminal is called a backward. The traffic direction from the terminal to the network is called a forward. The network operates to monitor the forward traffic for checking if the rate is the ACR or lower. If the traffic at the ACR or more is sent to the network, the network is executed to impose a penalty on the traffic of the ACR or more (non-conformance cell) at the network gate. The penalty contains an action of lowering a priority of the non-conformance cell or abandoning the cell itself.
This traffic monitoring and penalty handling are called a conformance check as described in TM 4.0. This conformance check is executed at the inlet of the unit called a UPC (simply called a UPC unit) based on the proper traffic parameter. Some of the UPC units treat a CBR or VBR. Herein, the description has been concerned with the UPC unit for treating the ABR. The traffic is abandoned if the nonconformance cell is sent out, so the terminal cannot keep the communications quality. To avoid this, the terminal tries to keep the ACR.
On the other hand, the network guarantees the regulated communications quality against the traffic at the ACR or lower (conformance cell) in the forward direction. The terminals termed herein contain a general terminal of an information source, a VSVD regulated in TM 4.0, and the other general sources viewed from the network such as a router and a LAN, which are all called a terminal. If the information source belongs to a network promoted by another mother body, the UPC function is called NPC. Since the NPC is substantially identical to the UPC, herein, the NPC and the UPC are both called a UPC or UPC's.
The ACR in the backward from the network to the terminal is temporarily stored in the UPC when the ACR passes through the UPC. The stored ACR reaches the terminal, where the ACR is effective as a monitoring rate for the UPC on the timing when the data reaches the UPC. The time taken in the process of UPC's receipt of the new ACR, passage of the ACR to the terminal, and arrival of the data at the ACR from the terminal to the UPC through the network is called .tau..sub.2. For example, it is assumed that the reciprocation of data between the UPC and the terminal has a delay of 2 msec, the ACR of the UPC is set as 10 Mbps, and the traffic sent from the terminal is monitored if it is 10 Mbps or lower. When receiving the new ACR=5 Mbps from the network, the UPC operates to set the ACR as 5 Mbps 2 msec later than the receipt and then monitor if the traffic from the terminal is 5 Mbps or lower.
In actual, however, the delay between the UPC and the terminal is not always constant. It may be a statistical queuing delay. For example, it is assumed that the statistical delay minimum is .tau..sub.3 and the maximum is .tau..sub.5. The adoption of .tau..sub.5 as .tau..sub.2 permits the UPC to let the nonconformance cell pass if the queuing delay is zero. Conversely, the adoption of .tau..sub.3 as .tau..sub.2 permits the UPC to erroneously treat the conformance cell if the cell suffers from the queuing delay on the way to the UPC.
The foregoing conventional UPC unit has a constant value of .tau..sub.2 set independently of the congestion. If .tau..sub.2 takes a large value, therefore, the UPC unit lets the nonconformance cell pass to the network if the network is in congestion, thereby increasing the congestion and lowering the communications quality. Conversely, if .tau..sub.2 takes a small value, the UPC unit lets even the doubtful cells be forcibly abandoned if the network is in congestion, thereby lowering the communication quality.